1. Field of the Invention
The present invention relates to an image forming apparatus that forms toner images on a sheet, and in particular relates to color shift correction technology that corrects shifts in the transfer position of a toner image from its regular position.
2. Description of the Related Art
In conventional image forming apparatus that include a multicolor printing function, photoconductor drums are disposed in four image generating units that are disposed in tandem, for example, and electrostatic latent images of the color components of yellow, magenta, cyan and black are generated on the photoconductor drums. In this case, a fixed scanning head or the like is used to generate the electrostatic latent images of the respective color components on the photoconductor drums on the basis of data of the respective color components, the electrostatic latent images are developed to generate toner images of the respective color components, and the toner images are sequentially superposed on a transfer belt. The superposed toner images (hereinafter called a composite toner image) are then transferred to printing paper that is conveyed. In this case, whether or not the positions of the toner images of the respective color components to be superposed on the transfer belt have shifted from their regular positions is checked. If there are positional shifts, control is conducted to reduce the positional shifts of the toner images because color shift will be generated. This correction control is done, for example, by changing the exposure timing in the main scanning direction and the sub-scanning direction in accordance with the detected shifts when image data is supplied from a memory to the fixed scanning head.
Technologies that have been proposed for the purpose of eliminating such shift are disclosed in (1) JP-A-8-278680 (regarding a color shift eliminating method), (2) JP-A-8-101555 (regarding a method of preventing the detection of a test pattern from becoming unable to be precisely conducted due to fluctuation in the output of a registration sensor resulting from the transfer belt), and (3) JP-A-8-258340 (regarding the prevention of color shift when the toner images are superposed).
The basic portions of the aforementioned technologies will be briefly described with reference to FIGS. 3 and 4. In this image forming apparatus, a registration sensor 81L at the front side of a transfer belt 90 and a registration sensor 81R at the rear side of the transfer belt 90 are disposed at the downstream side of the transfer belt as shown in FIG. 3 with a predetermined gap G between them in the main scanning direction (direction perpendicular to the moving direction of the transfer belt) of the toner images to be transferred. The registration sensors 81L and 81R read test patterns 80L and 80R (see FIG. 4) generated in order to detect the aforementioned shifts, detect the extent to which the positions of the test patterns have shifted from their regular positions (reference positions), and conduct control to reduce the shifts. In this example, when there are no shifts, as shown in FIG. 3 and the magnified view of FIG. 4, the test patterns 80L and 80R transferred to the transfer belt 90 are configured by first and third linear portions that are perpendicular to the moving direction of the transfer belt 90 and by second and fourth linear portions that intersect the first and third linear portions at a 45° angle.
In this case, the test patterns generated by the image generating units that generate the toner images of the color components of black, cyan, magenta and yellow (indicated by the subscript letters K, C, M and Y in FIG. 3) are transferred onto the transfer belt as shown in FIG. 3. Thus, the timing at which the first and third linear portions and the second and fourth linear portions cross the registration sensors 81L and 81R is detected, and the moving speed of the transfer belt 90 is referenced to calculate distances LTK, RTK, LVK and RVK relating to the color component of black.
Distances LTC, RTC, . . . , RTY relating to the color components of cyan, magenta and yellow are calculated in the same manner. Thus, using for example the first and third linear portions of black as references, distances a and a′, b and b′, and c and c′ to the first and third linear portions of the respective color components are calculated. In the case of FIG. 3, it will be understood that the generation of the yellow image is slanted by an angle θY with respect to the transfer belt 90 because the distance c and the distance c′ are different. Correction of the positional shift of this image (for details on the correction, see aforementioned (1) JP-A-8-278680) becomes implementable, and alignment of the toner images of the respective color components can be precisely conducted.
The aforementioned alignment control relating to the toner images is configured by a former-half process, such as generating the test patterns and acquiring the data resulting from the test patterns, and by a latter-half process for correcting the positional shifts of the images on the basis of processing of the acquired data and conducting printing where the shifts are eliminated. These processes are shown in FIG. 5 together with their required times. That is, step SA, which is a preparatory step for stabilizing the operation of the laser, requires about 43 seconds. Step SB, which is for printing the test patterns and acquiring data obtained as a result of the test patterns being read by the registration sensors in the alignment control, requires about 13.52 seconds. Step SC, which is for correcting printing based on the data acquired in step SB, requires about 6 seconds. Step SD, which is for checking the quality of alignment after correction, requires 8.48 seconds although it is similarly included in the correction. Step SE, which is processing following the end of step SD, requires about 6 seconds.
As described above, in conventional image forming apparatus, test patterns corresponding to the respective color components are generated in the image generating units that generate toner images of the color components of black, cyan, magenta and yellow. The generated test patterns are read by the registration sensors, and processing of programmed alignment control is conducted, whereby the positions of the toner images to be superposed and transferred can be precisely matched. Thus, color shifts in the composite toner image can be eliminated.
However, the aforementioned alignment control of the toner images is conducted when the power of the image forming apparatus is turned ON or when the warm-up of the image forming apparatus ends, or is executed when it is detected during printing that printing has reached a predetermined number of sheets and printing is temporarily stopped. This execution requires an amount of time of 1 minute or more when the aforementioned amounts of time and other amounts of time are considered. Consequently, there is the problem that efficiency is made worse because the amount of time during which printing cannot be conducted is increased in either case.